[lldb] Fix crash in CxxModuleHandler when std module is empty

We currently don't handle the error in the Expected we get
when searching for an equal local DeclContext. Usually this can't
happen as this would require that we have a STL container and
we can find libc++'s std module, but when we load the module in
the expression parser the module doesn't even contain the 'std'
namespace. The only way I see to test this is by having a fake
'std' module that requires a special define to actually provide
its contents, while it will just be empty (that is, it doesn't
even contain the 'std' namespace) without that define. LLDB currently
doesn't know about that define in the expression parser, so it
will load the wrong 'empty' module which should trigger this error.

Also removed the 'auto' for that variable as the function name
doesn't make it obvious that this is an expected and not just
a optional/ptr (which is how this slipped in from the start).

llvm-svn: 374525